1. Field of the Invention
The present invention relates particularly to an image forming apparatus using an electron source.
2. Description of the Related Art
Hitherto, there are known two types of electron emitting devices, i.e., a thermionic cathode and a cold cathode. Of these two types, known examples of the cold cathode include a surface conductive type electron emitting device, a field emission type electron emitting device (referred to as xe2x80x9cFE typexe2x80x9d hereinafter), and a metal/insulator/metal type electron emitting device (referred to as xe2x80x9cMIM typexe2x80x9d hereinafter).
Some examples of the surface conductive type electron emitting devices are described in M. I. Elinson, Radio Eng. Electron Phys., 10, 1290(1965) and other papers mentioned below.
A surface conductive type electron emitting device utilizes a phenomenon that electron emission occurs when an electric current is supplied to a small-area thin film formed on a substrate so as to flow parallel to the film surface. Surface conductive type electron emitting devices known so far employ an SnO2 thin film, as reported by M. I. Elinson et al., an Au thin film [see, e.g., G. Dittmer: xe2x80x9cThin Solid Filmsxe2x80x9d, 9, 317(1972)], an In2O3/SnO2 thin film [see, e.g., M. Hartwell and G. G. Fonstad: xe2x80x9cIEEE Trans. ED conf.xe2x80x9d, 519(1975)], a carbon thin film [see, e.g., Hisashi Araki et al.: Shinku (Vacuum), vol. 26, No. 1, 22(1983)], etc.
As a typical example of one of those surface conductive type electron emitting devices, FIG. 12 shows a plan view of the device reported by M. Hartwell et al.
Referring to FIG. 12, numeral 3001 denotes a substrate and 3004 denotes a conductive thin film of a metal oxide formed by sputtering. As shown, the conductive thin film 3004 is formed into an H-shape as viewed from above. An electron emitting portion 3005 is formed by carrying out an energization process to be described later, called xe2x80x9cenergization formingxe2x80x9d, on the conductive thin film 3004. A spacing L shown in FIG. 12 is set to 0.5-1 mm and a width W is set to 0.1 mm. Note that although the electron emitting portion 3005 is shown as having a rectangular shape at the center of the conductive thin film 3004, the drawing has been illustrated for the sake of easier understanding and does not exactly express the exact position and shape of electron emitting portions actually physically produced.
Known FE type electron emitting devices are reported, for example, by W. P. Dyke and W. W. Dolan, xe2x80x9cField Emissionxe2x80x9d, Advance in Electron Physics, 8, 89(1956) and C. A. Spindt, xe2x80x9cPhysical properties of thin-film field emission cathodes with molybdenum conesxe2x80x9d, J. Appl. Phys., 47, 5248(1976).
As a typical example of a construction of a FE type electron emitting device, FIG. 13 shows a sectional view of the device reported by C. A. Spindt et al.
Referring to FIG. 13, numeral 3010 denotes a substrate, and 3011 denotes an emitter wire made of a conductive material. Numeral 3012 denotes an emitter cone, 3013 denotes an insulating layer, and 3014 denotes a gate electrode. In the FE type device, field emission occurs from the top of the emitter cone 3012 by applying an appropriate voltage between the emitter cone 3012 and the gate electrode 3014.
As another example of a FE type device construction, there also is known a planar structure wherein an emitter and a gate electrode are arranged on a substrate, and lay substantially parallel to a flat surface of the substrate, rather than as shown in FIG. 13.
A known MIM type electron emitting device is reported, for example, by C. A. Mead, xe2x80x9coperation of Tunnel-emission Devicesxe2x80x9d, J. Appl. Phys., 32, 646(1961).
A typical example of a construction of the MIM type electron emitting device is shown in a sectional view of FIG. 14. Referring to FIG. 14, numeral 3020 denotes a substrate, and 3021 denotes a metal lower electrode. Numeral 3022 denotes a thin insulating layer having a thickness of about 10 nm, and 3023 denotes a metal upper electrode having a thickness of about 8-30 nm. In the MIM type device, electron emission occurs from the surface of the upper electrode 3023 by applying an appropriate voltage between the upper electrode 3023 and the lower electrode 3021.
Any of the cold cathodes described above do not require a heater for heating the devices because the cold cathodes can produce an electron emission at a lower temperature than that needed in the thermionic cathode. Therefore, a cold cathode can be formed with a simpler structure and a finer pattern than a thermionic cathode. Also, when a large number of cathodes are arrayed on a substrate with a high density, a problem such as thermal fusion of the substrate is less likely to occur. Further, a cold cathode has a high response speed, whereas a thermionic cathode has a low response speed because it starts operation upon heating by the heater.
For those reasons, studies regarding applications of cold cathodes have been actively conducted.
As to applications of the electron emitting devices, image forming apparatuses such as an image display unit and an image recording apparatus, charged beam sources, etc., have been studied.
Applications of the electron emitting devices to image forming apparatuses are disclosed in, for example, U.S. Pat. Nos. 5,532,548, 5,770,918 and 5,903,108, WO Nos. 98/28774 and 99/03126, as well as Japanese Patent Laid-Open Nos. 01-241742, 04-094038, 04-098744, 04-163833 and 04-284340.
Of those image forming apparatuses employing the electron emitting devices, attention often is focused on a flat display which has a thin body contributing to saving space, and which also is lightweight and expected to be eventually substituted for a CRT type display.
FIG. 20 is a perspective view schematically showing a partially uncovered flat image forming apparatus (airtight container) that employs an electron source comprising a number of electron emitting devices arrayed in the form of a matrix. In FIG. 20, numeral 27 denotes an electron emitting device of any type described above, and numerals 23 and 24 denote wires connected to the electron emitting device 27. Numeral 1 denotes a rear plate on which the electron emitting devices are arrayed, 20 denotes an image forming member made up of a phosphor, etc., and 19 denotes a metal film (metal back) to which a high voltage (Hv) is applied for irradiating electrons emitted from the electron emitting devices towards the image forming member. Numeral 11 denotes a face plate on one side of which the image forming member is arranged, and 4 denotes a support frame which, together with the face plate 11 and the rear plate 1, constitutes an airtight container 100. An inner space of the airtight container 100 is held in a vacuum state at a level of about 10xe2x88x924 Pa (Pascal).
The image forming apparatus described above has the following problems.
FIG. 15 is a partial schematic sectional view of a portion of the airtight container 100 (FIG. 20) constituting the above-described image forming apparatus.
Since the inner space of the airtight container 100 must be held in a vacuum state at a pressure level of about 1.3xc3x9710xe2x88x924 Pa as described above, some means for maintaining such a vacuum level is required. According to one conventional solution, an evaporable getter 8 filled with Ba is disposed together with a support 9 outside an image area, as shown in FIG. 15. After sealing off the vacuum container, Ba is scattered upon high-frequency heating, etc., to form a getter film, thereby holding the desired vacuum level substantially constant.
In FIG. 15, numeral 1 denotes a rear plate including an area (electron source area) 2 in which a number of electron emitting devices (not shown) are arrayed. Numeral 4 denotes a support frame, 11 denotes a face plate, and 12 denotes an image forming member made up of a film including a phosphor, etc., and a metal film (e.g., Al) called a metal back.
On the other hand, to accelerate electrons emitted from the electron emitting devices, a high voltage (Va) on the order of several hundred volts to several kvs is applied between the electron source area 2 and the image forming member 12. In an image display unit such as a display panel, the brightness level greatly depends on the amount of voltage Va applied. For achieving a greater brightness level, therefore, it is required to increase the applied voltage Va.
With an increase of the applied voltage Va, however, an electric field produced in the surroundings of the getter 8 and the support 9 (which are arranged outside the image area) is also increased. This increase of the electric field has raised a problem of the occurrence of a discharge at edges of both the getter 8 and the support 9 or at a boundary surface between the support 9 and the rear plate 1, where an electric field tends to enhance due to the shape of those components. The produced electric field is determined (as described later in greater detail) by electrical characteristics of various components.
In some cases, for the purpose of bearing the vacuum container against the atmospheric pressure, supports (spacers 101), each being formed of a relatively thin member, are provided in the image area between the rear plate 1 and the face plate 11. FIG. 17 shows a schematic perspective view of the airtight container 100 in which spacers 101 are disposed. In FIG. 17, portions of the face plate 11 and the support frame 4 are omitted for the sake of convenience. The same numerals in FIG. 17 as those shown in FIG. 20 denote the same components. Specifically, numeral 27 denotes an electron emitting device, 20 denotes a film made up of a phosphor, etc., 19 denotes a metal back, and these components 19 and 20 collectively form an image forming member. Also, numeral 24 denotes an upper wire connected to ends of respective electron emitting devices, and 23 denotes a lower wire connected to other ends of those electron emitting devices. Since the spacers 101 are disposed in an image area, spacer surfaces are exposed to a high electric field. Accordingly, in at least some conventional cases, a discharge phenomenon has occurred at the spacer surfaces.
For overcoming such a problem, it is proposed in some of the above-cited publications to remove charged electricity by processing the spacers 101 such that a small current is allowed to flow through each spacer 101.
Even with the processing of the spacers, however, it has been experienced, in at lest some cases, that longitudinal ends 110 of each spacer 101 cause a discharge at a lower voltage than in other portions. The reason for this discharge presumably is that the ends 110 of the spacer 101 are of a more complicated structure, and the contact of the spacer ends 110 to the face plate 11 and rear plate 1 tends to be unstable. Furthermore, although depending on the methods employed for manufacturing and handling the spacers 101, the spacer ends 110 tend to be more susceptible to micro-protrusions, cracks and other shape defects, and hence are more likely to become discharge sources than are other spacer portions. Suppressing the occurrence of discharge at the spacer ends 110, due to those factors, is very important in image display units.
Also, where the spacer end 110 located in the image area is obliquely cut, as shown in FIG. 18, this arrangement noticeably increases a probability that an electric field will enhance at an end 111 of the spacer on the side of the rear plate 1, and hence also increases the probability that a discharge will occur there. In the image display unit having such a structure, it is particularly important to suppress discharge from occurring at the spacer end 111 on the side of the rear plate 1.
Furthermore, in at least some cases, the spacer end 110 is arranged outside the image area as shown in FIG. 19, or the spacer end 110 is fixed to the rear plate 1 by using a support 102 as shown in FIG. 16. In any of these structures, it is also important to suppress any discharge that may occur due to the shape of the spacer end 110 and support 102.
Of four sides of the image area, even a side where structural components such as the getter support and the spacer support are not present outside the image area may undergo a similar problem. In other words, when the distance between the support frame 4 and the image area is reduced more and more for achieving a smaller size of the airtight container 100, surface discharge may occur at an inner surface of the support frame 4.
The term xe2x80x9csurface dischargexe2x80x9d, as used in this description, means a discharge phenomenon occurring between two conductive members along an insulator surface; i.e., a discharge phenomenon occurring between one conductive member on the face plate 11 and another conductive member on the rear plate 1 along the surface of the support frame 4 that is an insulator.
The above-mentioned discharge typically occurs abruptly during the image display operation. Once it has occurred, the discharge not only distorts an image, but also noticeably deteriorates an electron source area around a location where the discharge has occurred to such an extent that a desired display quality is no longer obtained, in at least some cases after the occurrence of the discharge.
In view of the problems set forth above, it is an object of the present invention to provide an image forming apparatus which can prevent a discharge from occurring outside an image area of a display device during an image display operation, and which can produce a displayed image having a high quality.
To achieve the above object, according to one aspect of the present invention, there is provided an image forming apparatus comprising (A) a first substrate; (B) a second substrate arranged in an opposing and spaced apart relation to the first substrate; (C) a support frame having an inner periphery forming a substantially rectangular shape, the support frame being arranged between the first and second substrates to surround a space between a principal surface of the first substrate and a principal surface of the second substrate, for maintaining the space in a depressurized condition; (D) a plurality of electron emitting devices arranged on the principal surface of the first substrate facing the space; (E) an image forming member having an outer periphery forming a substantially rectangular shape, the image forming member being arranged on at least a portion of the principal surface of the second substrate facing the space in an opposing relation to the plurality of electron emitting devices; (F) a spacer disposed in the space for maintaining a separation between the first and second substrates; and (G) a conductive film arranged on at least another portion of the principal surface of the second substrate facing the space. The conductive film surrounds, and is spaced apart from, the image forming member. The conductive film preferably is supplied with a potential lower than that applied to the image forming member. The spacer preferably has a length in the longitudinal direction thereof greater than that of the image forming member in the same longitudinal direction, each longitudinal end of the spacer preferably is arranged between the inner periphery of the support frame and a respective plane through which a conductive film extends, each respective plane preferably extends substantially perpendicularly to the principal surface of the second substrate.
To achieve the above object, according to another aspect of the present invention, there is provided an image forming apparatus comprising (A) a first substrate; (B) a second substrate arranged in an opposing and spaced apart relation to the first substrate; (C) a support frame having an inner periphery forming a substantially rectangular shape, the support frame being arranged between the first and second substrates to surround a space defined between a principal surface of the first substrate and a principal surface of the second substrate, for maintaining the space in a depressurized condition; (D) a plurality of electron emitting devices arranged on the principal surface of the first substrate facing the space; (E) an image forming member having an outer periphery forming a substantially rectangular shape, the image forming member being arranged on at least a portion of the principal surface of the second substrate facing the space in an opposing relation to the plurality of electron emitting devices; (F) a first conductive film arranged on at least another portion of the principal surface of the second substrate facing the space so as to surround, and be spaced apart from, the image forming member; and (G) a second conductive film connecting the first conductive film to the image forming member. The first conductive film preferably is supplied with a potential lower than that applied to the image forming member.
With the image forming apparatus of the present invention constructed as set forth above, the distance between the image forming member and the support frame can be shortened, and any electric field which imposes on structural components, such as the spacer ends and the spacer support member, can be weakened. As a result, an image forming apparatus is realized which can form a stable image with a high brightness level sustained for a long period of time, and which is lightweight and easy to manufacture.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.